The invention relates to a FM-receiver with transmitter characterization, comprising a tuning unit connected to an aerial input and to which there are connected, one after the other, an IF-amplifier, a FM-detector, a demodulation circuit for demodulating a discrete transmitter characterization signal, a clock regeneration circuit, a decoding device for decoding the discrete transmitter characterization signal and a signal processing unit, also comprising a pilot regeneration circuit connected to the FM-detector for regenerating a stereo pilot signal.
Such a FM-receiver with transmitter characterisation is disclosed in the publication "The SPI-system for FM-tuning", published in 1978 by N. V. Philips' Gloeilampenfabrieken, Electronic Components and Materials division.
The known FM-receiver has the possibility to display transmitter characterization signals. The transmitter characterization signals may, for example, contain information about the location of the transmitter to which the receiver is tuned, the broadcasting company and the nature of the transmitted program. This information is transmitted in the form of continuously repeated digital messages. Each message is preceded by a start code word which is used for the word-synchronization in the receiver. For the transmission, use is made of a subcarrier in the baseband of the FM-signal, whose phase is modulated with the digital messages.
In the known FM-receiver, the transmitter characterization signals are available in digital form at the output of the above-mentioned demodulator. For an adequate decoding of these digital transmitter characterization signals, alternatively denoted code signals, a clock signal is required which is in synchronism with the clock signal with which coding was effected in the transmitter. Regeneration of this clock signal is carried out in the clock regeneration circuit, in which, in the prior art receiver, the code signals themselves are used to generate the clock signal. To that end, the frequency of the code signals is doubled and applied as control signal to a phase-locked loop. The frequency of a voltage-controlled oscillator included in the phase-locked loop is equal to the desired clock frequency. Phase deviations, if any, are corrected in a phase control circuit.
In the case of a temporary absence or disturbance of the code signals, the control of the voltage-controlled oscillator is interrupted or disturbed, which may cause the clock-code-synchronization to get lost. Further processing of the code signals already stored is not possible then because of the fact that a correct clock signal is absent. Pull-in of the phase-locked loop at the frequency determined by the code signals after such an interruption or disturbance may require so much time, depending on the bandwidth and loop gain, that a relatively short disturbance in the reception of the code signals may result in rather long interference effects in the display of the transmitter characterization signals.